1. Field of the Invention
The present invention relates to a body frame structure of a two-wheeled motor vehicle provided with a swing arm type rear suspension.
2. Description of the Background Art
Many two-wheeled motor vehicles are provided with a so-called swing arm type rear suspension. That is, a swing arm for a rear wheel is secured to a body frame and at the same time it is suspended from the body frame through a rear suspension. Generally, once characteristics of a rear suspension are determined, a layout relation thereof (e.g. mounting position and inclination) to a swing arm is mostly determined. This form of rear suspension is shown in a "saddle type vehicle" as disclosed in Japanese Patent Laid-open No. Hei 2-74487.
According to FIGS. 1 to 3 and 6 of the publication, a pair of upper frame members are extended backward and downward from a head pipe 4, a front end portion of a rear fork is mounted between rear lower portions of the upper frame members 14, a rear end portion of a damper 8 is secured to a longitudinally intermediate position of the rear fork, and a front end portion of the damper 8 is attached to a cross member 15 mounted between the upper frame members 14. FIGS. 2 and 3 show the cross member 15 mounted between straight portions of the upper frame members 14 as seen from the side, an arm (with no reference numeral affixed thereto) is extended upwardly from the cross member 15 and the front end portion of the damper 8 is attached to an upper end of the arm, so that the damper 8 extends nearly in parallel with the above straight portions. Thus, the front end portion of the damper 8 is secured to the cross member 15 in an offset manner with the side view.
It is desirable that the center of gravity of the two-wheeled motor vehicle 1 be as low as possible, and it is therefore desirable that fuel tank 11 be disposed at a low position on the vehicle. The height of the fuel tank 11, which is disposed in front of a seat 12, is set so as to be appropriate in vehicular driving relative to the level of the seat 12. In view of these points it is also desired that the upper frame members 14 extending below the fuel tank 11 be positioned low on the vehicle.
In the above conventional technique, the upper frame members 14 are positioned relatively low and the damper 8 is disposed above the upper frame members 14 and substantially in parallel with the straight portions of the upper frame members with side view.
As shown in FIG. 2 of the above publication, the front end portion of the damper 8 is attached to the cross member 15 in an upwardly offset manner. Consequently, a torsional moment is induced in the cross member 15 due to the eccentric load of the damper 8. Thus, it is required that the rigidity of the cross member 15 be enhanced, which results in an increase in both weight and cost.
One measure for preventing such an inconvenience is to raise the height of the cross member 15 up to the level of the front end portion of the damper 9. This can be done by raising the height of the entire upper frame members 14 up to the level of the cross member 15. However, if the upper frame members 14 are raised, they will interfere with the edge of the fuel tank 11 or of the seat 12. To avoid this interference, the edge of the fuel tank 11 or of the seat 12 must be made wider. However, this is undesirable because an increase of the vehicle width will result.
Accordingly, in connection with a two-wheeled motor vehicle having a front end portion of a rear suspension damper 9 attached to an upper part of a body frame, it is an object of the present invention to provide a technique capable of (1) reducing the weight and cost of the body frame and (2) setting a fuel tank and a seat at optimum heights without changing their shapes and sizes.